1. Field of the Invention
The invention relates to polyurethane powder coating compositions (PUR PCCs) based on trans-1,4-cyclohexanediisocyanate (CHDI) adducts blocked by epsilon-caprolactam. These adducts serve as isocyanate components, wherein the PUR PCCs yield a matte surface after setting.
2. Discussion of the Background
Quite often a high gloss is not desired with a given coating. There may be numerous reasons for this; e.g., the fact that a high gloss surface may require much more cleaning than a matte surface; or safety considerations of avoiding glare.
The simplest principle by which a matte surface is obtained is admixture of filler materials, e.g., chalk, finely divided SiO.sub.2, or barium sulfate, into the PCC, in an amount correlated with the desired matte effect. However, such admixture is detrimental to the properties of the coating, e.g., adhesion, flexibility, impact strength, and chemical resistance.
PCCs based on cycloaliphatic diisocyanates and OH-group-containing polymers are known. The most industrially important of these are PCCs based on adducts of isophoronediisocyanate (IPDI).
Whether one begins with epsilon-caprolactam-blocked adducts of IPDI and chain-lengthening agents (see Ger. Nos. OS 21 05 777 and OS 25 42 191), or with isocyanate-group-containing IPDI adducts (see Ger. No. OS 27 35 497), after hardening one always obtains high gloss surfaces.
Also known are PCCs based on cycloaliphatic diisocyanate adducts which yield matte surfaces after setting. However, these are accompanied by various disadvantages. Thus, the uretdione-group-containing hardeners described in Ger. No. OS 33 28 133 are costly to produce and are temperature sensitive. Ger. No. OS 32 32 463 describes a complex 3-component mixture comprised of a carboxyl-group-containing isocyanate component, an alcohol component, and an epoxide.
It is known from Ger. Pat. App. No. P 35 36 017 that powder mixtures with epsilon-caprolactam-blocked trans-1,4-cyclohexanediisocyanate (CHDI) as a hardener yield high gloss PUR coatings after setting.